1. Field of the Invention
The present invention relates to mode detection in an initial synchronization process of an orthogonal frequency division multiplexing (OFDM) receiving apparatus, and more particularly, to an OFDM receiving apparatus and a mode detecting method thereof, where the OFDM receiving apparatus and a mode detecting method thereof perform accurate and quick mode detection by first detecting a fast Fourier transform (FFT) mode and then detecting a guard interval (GI) mode by using a result of detecting the FFT mode in the same signal, and the OFDM receiving apparatus has a small chip size.
2. Description of the Related Art
Generally, a broadcasting system of a high definition television (HDTV) compresses digital data at about 1 Gbps and obtained from a high definition image source into data in a range of about 15 to about 18 Mbps, and transmits digital data in tens of Mbps via a limited band channel in a range of about 6 to about 8 MHz.
As such, a modulation method used in the broadcasting system of the HDTV requires high band efficiency, since in the modulation method, the digital data in tens of Mbps is transmitted via the limited band channel. Also, since the HDTV uses a simulcast method using a channel in a very high frequency (VHF)/ultrahigh frequency (UHF) band that is assigned for analog TV, the HDTV must not be affected by channel interference due to an analog TV signal.
In order to increase the transmission efficiency per bandwidth and prevent interference, an orthogonal frequency division multiplexing (OFDM) method from among digital modulation methods is used as a next generation HDTV broadcasting method. In the OFDM method, a symbol column received in series is converted into parallel data in a predetermined block unit, and parallelized symbols are multiplexed to different subcarrier frequencies.
The OFDM method uses a multicarrier, wherein carriers of the multicarrier are orthogonal to each other. When a result of multiplying two carriers is 0, the two carriers are orthogonal to each other, and such orthogonal carriers are used to increase the spectrum efficiency since spectrums of the carriers overlap.
In the OFDM method, a receiver synchronizes with a transmitter in order to extract digital data from a modulated signal. During such a synchronization process between the receiver and the transmitter, a fast Fourier transform (FFT) mode and a guide interval (GI) mode are detected from a received signal.
As described above, before the transmitter transmits data, an inverse FFT is performed on the data, and thus the receiver demodulates a received signal via a FFT. Accordingly, the receiver is able to determine a starting point and an effective data period of a symbol on which the FFT is to be performed.
The starting point and the effective data period of the symbol differ according to a FFT mode and a GI mode of a system. Here, the FFT mode is an interleaving method performed to minimize effects caused by an error that may be generated while transmitting and receiving a signal. For example, the FFT mode is determined to be one of 2K, 4K, and 8K modes.
Also, the GI mode is based on a length of a protection period that is inserted between each symbol in order to prevent interference between the symbols. The protection period is prepared by copying lower predetermined bit values of data in the effective data period. Here, the GI mode is determined to be one of ¼, ⅛, 1/16, and 1/32 modes, based on a length of the protection period, i.e., the length of lower bits of effective data.
However, a lot of time is consumed for the receiver to detect a mode of a system. Also, since each mode is detected by using signals received via different channels, an error may occur while detecting a mode.